1. Field of the Invention
The present invention relates to a motor control device for controlling the operation of a motor, and more particularly to a motor control device with a capability of detecting synchronization failure. The present invention also relates to a motor drive system incorporating such a motor control device.
2. Description of Related Art
There have conventionally been developed motor control devices (position-sensorless control devices) that estimate the rotor position of a motor without the use of a rotor position sensor and controls the motor based on the so estimated rotor position. In motor control devices of this type, an abrupt variation in the load of the motor may cause synchronization failure. If synchronization failure occurs, the motor may come to a standstill and become uncontrollable. Thus, techniques that can surely detect synchronization failure have been sought.
There have been proposed various techniques for detecting such synchronization failure. Many of them detect synchronization failure by paying attention to a lowering in power factor, or ineffective current, resulting from a current that does not flow in normal operation.
For example, JP-A-2001-025282 discloses a technique whereby synchronization failure is detected through comparison of the period of ineffective current and the period of its voltage in case of a standstill resulting from synchronization failure.
For another example, a technique is known whereby synchronization failure is detected when the deviation between the estimated speed as estimated by a speed estimator and the specified speed has remained equal to or greater than a predetermined value for a predetermined period or longer. These conventional techniques, however, have the disadvantage that, in particular conditions, they cannot detect synchronization failure satisfactorily (for example, they simply cannot detect synchronization failure at all).
Incidentally, JP-A-H11-018499 discloses a technique whereby synchronization failure is detected by estimating and constantly comparing the γ-axis and d-axis induction voltages that are generated as the rotor of a motor rotates. On the other hand, JP-A-2004-064902 discloses a technique whereby, to detect synchronization failure of a synchronous motor, the voltage to be specified is determined based on the motor generation coefficient and the specified excitation current, then a voltage correction amount is produced to make the specified excitation current and the actual excitation current equal, and then the voltage correction amount is compared with a predetermined value.
Thus, while, as described above, techniques that can surely detect synchronization failure have been sought in motor control devices that control a motor without the use of a rotor position sensor, none of the conventionally known techniques can satisfactorily meet the requirements.